The didemnins form a class of cyclic depsipeptides which have been isolated from various species of the Trididemnum genus. They have been shown to have potent activity against viruses and tumor cells (Rinehart, Jr., et al., J. Am. Chem. Soc., 103, 1857-59 (1981). Didemnin B, up to now the most active compound of this class, has been shown to have potent immunosuppressive activity (Montgomery et al., Transplantation, 40, 49-56 (1985) and a more potent inhibition of binding of prolactin to human lymphocytes than other didemnin compounds (Montgomery et al., Fed. Prac., 44, 634 (1987).